1. Field of the Invention
The present invention relates, in general, to an apparatus and method for flaw discrimination, and in particular, is directed to an apparatus and method for discriminating flaw depths of 5% and 10% of the tube wall thickness of electric resistance welds (ERW).
2. Description of the Related Art
When tube is manufactured from flat stock, the stock is bent into a cylindrical shape and joined by a weld to form a longitudinal seam therein. It is important to examine this weld line for flaws such as cracks and lack of weld fusion and to properly reject pipes with flaw depths exceeding specified limits.
It is known in the art to inspect the weld line in the final inspection of electric resistance welded tubing by flux leakage and conventional ultrasonics. Most ultrasonic techniques for flaw detection are based upon changes in reflected amplitude as a function of throughwall depth from shear waves alone. The limited information obtained is not reliable due to the complexity of the interaction between the ultrasonic wave and the flaw. This complex interaction is dependent upon the position of the flaw (i.e., outer diameter (OD), inner diameter (ID), or midwall), the orientation of the flaw, and other geometrical factors such as flaw shape and roughness.
U.S. Pat. No. 3,868,847 discloses an apparatus that fits inside a tube for inspecting an elongated weld such as a seam weld with ultrasonics. There are longitudinal wave ultrasonic transducers provided for inspecting the weld along the thickest portion. There are also employed shear wave ultrasonic transducers for inspecting the boundary zones between the weld metal and the plate being welded.
U.S. Pat. No. 4,658,649 describes an ultrasonic method and apparatus for detecting and measuring defects in metals with the use of longitudinal and shear wave modes. A longitudinal mode wave and a shear mode wave are propagated within the object and the shear wave is converted into longitudinal mode waves by reflection from the opposite surface of the object. The propagated and mode converted waves are reflected from the different portions of the defect and the echoes arrive serially in time at a receiver transducer.
U.S. Pat. No. 4,289,030 describes a method for detecting a flaw proximate to a welded seam in a tube. An electromagnetic acoustic transducer (EMAT) generates a horizontally polarized shear wave in the wall of the pipe. The pipe is monitored to detect a reflected horizontally polarized shear wave, and a time-dependent representation of the amplitude of the reflected wave is displayed. The wave generating, monitoring, displaying steps are repeated along a length of the tube to provide a comprehensive flaw inspection of that length of the pipe.
U.S. Pat. No. 4,627,289 relates to a method for ultrasonic flaw detection of an electric resistance welded steel tube. An ultrasonic wave is projected with a frequency range of from 25 MHz to 500 MHz at an angle of incidence from 0.degree. to 12.degree. onto the weld zone of the tube. Additionally, another ultrasonic wave with a frequency of from 2 MHz to 10 MHz at an angle of incidence from 15.degree. to 27.degree. is projected onto the same weld zone to enable discrimination between cold weld defects and other defects such as inclusions and penetrators.
Ultrasonic examination techniques employing an array of transducers are also well known in this art, as taught in U.S. Pat. Nos. 4,718,277; 4,660,419; 4,641,531; 4,541,064; 3,828,609; and 4,803,486.
Other references related to nondestructive evaluation include U.S. Pat. Nos. 4,679,437; 3,921,440; and Reissue Patent No. RE. 30,926.
In the final inspection of electric resistance welded tubing, there is a need to maintain high inspection speeds, good quality assurance, and the ability to discriminate between flaw depths of 5% and 10% of the tube wall thickness. The ideal inspection technique would provide 100% detection of all flaws having a throughwall depth of 10% or greater while not rejecting flaws having depths of 5% or smaller. By adapting these criteria the code requirements for inspection may be met, and the needless rejection of insignificant flaws may be avoided together with the resulting waste in time and materials.